Zirconium or hafnium tetrachloride is reduced to zirconium or hafnium metal by reaction with a metal such as magnesium or sodium. The reaction is normally accomplished in a heated reduction vessel. This vessel is normally under an argon atmosphere and may become positively or negatively pressurized depending on the temperature changes which occur during the reduction reaction. The reaction chamber is protected from excessive increased or reduced pressure by a molten metal seal referred to as a molten metal vacuum pressure relief seal. Conventionally such a seal is designed to be an integral part of the lid. In conventional practice, a molten lead or lead alloy seal is provided. The use of lead and lead alloys has, however, created several operating problems with the reduction reactor. First, it is important that the seal not be frozen, i.e., to operate properly the lead or lead alloy should be molten. This might require supplemental heating of the metal used in the seal depending on the operating temperature of the reactor and the seal housing. Secondly, excessive negative pressure can draw liquid metal from the seal into the reaction vessel both breaking the seal to the atmosphere which could introduce unwanted air as well as contaminating the reaction mixture with molten lead or lead alloy. Lastly, waste disposal of used seal material becomes a problem since the lead and lead alloys used form a toxic waste producing special disposal problems. Background literature describing the prior practices provides further insights into the operation of such reaction vessels and the problems associated with liquid lead and lead alloy seals and the following references are incorporated by reference herein in their entirety.